High-power lasers are typically characterized by inferior beam quality, stability, and heat dissipation, as compared to that of lower power lasers. Combining several low-power lasers by incoherently adding the field distributions of several laser output beams results in that the combined beam-quality factor M2) is relatively poor with low optical brightness. When the field distributions are coherently added, with the proper phase relations, the combined beam quality factor can be as good as that of one low-power laser, while the combined power is greater by a factor equal to the number of the lasers.
When coherently combining two or more laser output fields, two major difficulties are encountered. The first results from the need for proper coupling between the individual laser fields, so as to enable relative phase locking between them. Such coupling typically introduces excessive losses to each laser field, and requires very accurate relative alignment. The second (and somewhat related) difficulty results from the need for accurately controlling the relative phase between the different laser fields, so as to ensure constructive interference between them. This requires that the distances between the participating optical components must be very accurately controlled, causing the output power to be extremely sensitive to thermal drifts and acoustic vibrations.
Attempts have been made to obtain high power concomitantly with good beam quality based on intra-cavity phase locking and coherent addition of lasers [1-7]. Several techniques dealing with intra-cavity coherent addition of single transverse mode (TEM00) laser beams in fiber lasers have been developed [8-11]. According to these techniques, the phase locking and coherent addition is accomplished by the use of fiber couplers. Single transverse mode fiber couplers (2×2 for example) have been used recently to obtain intra-cavity coherent addition of single transverse mode (TEM00) laser beams in a fiber laser configuration [8-11]. Here, one of the output terminals of the 2×2 standard single mode fiber coupler was angle spliced so that no reflection from that terminal is present. The resulting coupler's operation when placed in a resonator is similar to that of a 50% beam splitter. This approach, however, is applicable only in fiber laser systems, and designed only for single TEM00 beams.
U.S. Pat. No. 3,414,840 discloses a technique of synchronization of power sources. Here, two laser oscillators are used, each including a first mirror and a laser medium and each sharing in common a second mirror, and means for extracting wave energy from the oscillators. The first mirrors and the common second mirror form a pair of resonant cavities. A 3 db hybrid junction, having two pairs of conjugate ports located within a common region of the cavities, is used for coupling wave energy among the mirrors and out of the cavities. The laser medium for each oscillator is located between one of the first mirrors and one port of one of the pairs of conjugate ports. This arrangement utilizes discrete beam splitters within the resonator in order to coherently add two or more laser channels, operating in the TEM00 transverse mode, and obtain a single transverse and longitudinal mode (single frequency) output beam.
Techniques have also been developed for external coherent combining of two lobes of a transverse high order mode distribution emerging from a laser [12-13].